Thin-film devices based on superconducting materials have demonstrated superior performance for many applications. However, they will function properly only at deep cryogenic temperatures, below the critical temperature Tc of the given superconducting material. Conventional low-temperature superconductors (LTS) have been known for almost 100 years, and thin-film circuits based on the metallic element niobium (Nb) can be built around active devices known as Josephson junctions (JJ). These junctions form the basis for active superconducting electronic elements, both analog and digital. More specifically, large numbers of JJs can be integrated together using an ultra-fast circuit low-power technology known as RSFQ (rapid-single-flux-quantum) logic, which has provided digital integrated circuits (ICs) that are clocked at 100 GHz or more. However, these Nb-based circuits require cooling to below Tc=9 K to function. This is technologically feasible using closed-cycle refrigerators known as cryocoolers, but operation at higher temperatures would tend to have improved power efficiency and easier system packaging.
LTS Nb circuits are comprised of polycrystalline thin films, fabricated on an ambient-temperature substrate by sputtering techniques and patterned into ICs. The structure typically includes several superconducting wiring layers, separated by thick insulating layers. It also include Josephson junctions (JJs), typically comprised of an SIS tunnel-junction trilayer, with a thin insulator I (of order 1 nm thick) separating two superconducting S layers. This permits weak electronic tunneling between the two superconducting layers that may be modulated by magnetic field or voltage.
In 1986 and 1987, a new class of “high-temperature superconducting” (HTS) materials was discovered, based on a crystal structure with parallel copper-oxide (cuprate) planes. The most practical material of this class is YBa2Cu3O7 (also known as YBCO), with Tc=92 K. Unfortunately, conduction in YBCO and related cuprate materials is highly anisotropic, with severe problems conducting across grain boundaries. For this reason, practical IC processes require multi-layer thin films that are highly epitaxial with few grain boundaries or other defects, and are practically single crystals. That, combined with the high fabrication temperatures (>700° C.) needed to assure crystalline quality, makes device fabrication difficult. While simple RSFQ circuits operating at high speeds have been demonstrated in the laboratory, it has proven very difficult to develop a reliable IC process for YBCO.
An SIS structure for YBCO is not compatible with the high-temperature processing, since the barrier is too thin to maintain its integrity. An alternative JJ structure in the prior art is SNS, where N refers to a non-superconducting resistive material, which may be from 10 to 100 nm thick, depending on the material. See, for example, U.S. Pat. No. 5,468,973. For YBCO superconductors, SNS junctions have been made with horizontal ramp-edge structures, due to the anisotropy of conduction in the cuprate films. See, for example, U.S. Pat. No. 6,066,600. However, it has been difficult to develop a uniform process for YBCO JJs compatible with high-density ICs.
Researchers have continued to search for alternative materials that may exhibit superconductivity at relatively high temperatures (perhaps even exceeding the cuprates), but may also be easier to fabricate and process. Very recently, a material was fabricated having a crystal structure shown in FIG. 1A, comprised of atomic layers of iron-arsenic (FeAs) alternating with atomic layers of rare-earth oxides, with composition such as LaFeAsO. Superconductivity in this compound with Tc=26 K was first reported by Kamihara at the end of February 2008, by partially substituting fluorine for oxygen to “dope” the material with carriers. Y. Kamihara, et al., “Iron-Based Layered Superconductor La[O1-xFx]FeAs (x=0.05-0.12) with Tc=26 K”, J. Am. Chem. Soc., 130 (11), 3296-3297, Feb. 23, 2008. pubs.acs.org/cgi-bin/abstract.cgi/jacsat/2008/130/ill/abs/ja800073m.html (expressly incorporated herein by reference). Subsequent research in China in March and April (Ren, et al.) verified these findings and achieved T, up to 55 K by substitution of another rare-earth element samarium (Sm) for lanthanum (La). Z. A. Ren, et al., “Novel Superconductivity and Phase Diagram in the Iron-based Arsenic-oxides RFeAsO1-δ (R=rare earth metal) without F-Doping”, preprint submitted on Apr. 16, 2008. arxiv.org/abs/0804.2582, published in Europhysics Letters, vol. 83, p. 17002, July 2008 (expressly incorporated herein by reference). Within this application, all these materials are collectively identified as RFAO, where R refers to one of the rare-earth elements (as described further below). Research exploring further substitutions that may increase Tc even further are continuing worldwide.
It is believed that the superconductivity is associated with conduction in the parallel Fe layers, which would be expected to exhibit anisotropic behavior, similar to that in YBCO and related cuprates. It is somewhat surprising that the superconductivity is associated with Fe atoms, since it is well known in the prior art that Fe atoms are generally magnetic, and that magnetic atoms generally degrade superconductivity.
Following the initial series of discoveries, several other closely related families of superconductors were discovered. The initial RFAO materials are sometimes referred to as “1111” compounds, for the ideal stoichiometries of the crystal structure. Another family is identified as “122”, with ideal structure AeFe2As2, with Ae refers to alkaline earth atoms such as Ba, Sr, and Ca. See, for example, M. Rotter, et al., “Superconductivity at 38 K in the Iron Arsenide (Ba1-xKx)Fe2As2”, Physical Review Letters, vol. 101, p. 107006, September 2008 (expressly incorporated herein by reference). Again, this consists of FeAs layers, separated by Ae layers (see FIG. 1B). These 122 materials exhibit a superconducting critical temperature as high as 38 K, and generally need to be doped by substitution, either of alkalai atoms (K, Na) on the Ae site (for hole doping) or of Co atoms on the Fe site (for electron doping). There is also a “111” family with general formula AFeAs, with A an alkali atom (Li or Na), with Tc up to 25 K. See, for example, C. W. Chu, et al., “Synthesis and characterization of LiFeAs and NaFeAs”, arxiv.org/abs/0902.0806, submitted February 2009 (expressly incorporated herein by reference). In all cases, the superconductivity seems to be associated with the Fe layers.
While the RFAO (1111) materials still exhibit the highest values of Tc of the FeAs families (around 56 K as of April 2009), the other families of materials may still be of interest if they offer advantages in materials processing or other properties such as device reproducibility.
Finally, another family of superconductors that may be closely related to the FeAs compounds has recently been reported, based on FeSe, where other chalcogenides (S and Te) may be substituted on the Se site. Although the superconducting critical temperatures of these materials at ambient pressure are modest (˜10 K), large hydrostatic pressure has been shown to cause Tc to increase to as high as 37 K. See S. Margadonna, et al., “Pressure evolution of low-temperature crystal structure of 37 K Tc FeSe superconductor”, arxiv.org/abs/0903.2204, Mar. 12, 2009 (expressly incorporated herein by reference). If the high-pressure phase can be stabilized in thin film form, this material may also be practical for superconducting devices.
Most FeAs superconducting samples heretofore reported are comprised of compressed polycrstalline powders, or of very small single crystals, which are not optimized for electronic applications. However, a superconducting thin film of FeAs compounds prepared by pulsed-laser deposition was very recently reported. See H. Hiramatsu, et al., “Superconductivity in Epitaxial Thin Films of Co-Doped SrFe2As2 with Bilayered FeAs Structures and their Magnetic Anisotropy”, Applied Physics Express, vol. 1, p. 101702, September 2008 (expressly incorporated herein by reference). Pulsed laser deposition was used early in the development of cuprate superconducting materials (see, for example, U.S. Pat. No. 5,290,761), but eventually found to be unsuitable for superconducting device manufacturing due to embedded particulates and difficulties in uniformity and scaling.